This invention relates to the detection of gases using laser beams. More particularly this invention relates to the remote detecting of gases in the atmosphere using frequency-mixed radiation from CO.sub.2 lasers.
The detection of gases in the atmosphere using laser beams in the infared region is known to those skilled in the art. Canadian Patent No. 808,760 describes the detection of hydrocarbon gases using noble gas lasers such as a helium-neon laser mounted in an aircraft. Briefly, the method comprises the use of two laser beams of slightly different wavelengths, either from the same laser or two lasers. One preselected wavelength is highly absorbed by the gas to be detected while the other is not thereby providing a differential coefficient. The laser beams pass through the gas in question and are reflected back to a common detection source which measures the intensity of the two beams. Any difference in the measured intensities determines the presence and quantity of the gas in question. Dust, water droplets and other light scattering materials in the atmosphere act in similar manner on the two beams and, thus, are factored out.
While such a detection scheme should be highly satisfactory in determining the presence or absence of preselected gases, in practice it is not, due to the restrictive number of wavelengths emitted by such lasers and the number of interfering gases possibly present in the atmosphere either singly or in combination. For example, the most popular and frequently used of such lasers, the helium-neon laser, only emits at 10 possible wavelengths. While these particular wavelengths, for example, have been found to be selective with respect to methane, they are not useful for detection of ethane.
Other laser beams may be substituted for the helium-neon laser to permit selective detection of other gases such as ethylene which cannot be detected satisfactorily with the helium-neon laser. Two of the present inventors published an article entitled "Remote Measurement of Ethylene Using a CO.sub.2 Differential-Absorption Lidar" by E. R. Murray and J. E. van der Laan in Applied Optics, Volume 17 at page 814 (Mar. 1, 1978). This article describes the detection of ethylene gas in the atmosphere by selective absorption of wavelengths emitted by a CO.sub.2 laser.
It is also known that the number of wavelengths emitted by a laser source can be increased as well as the frequency range changed by the use of doubling and mixing crystals. N. Menyuk and G. W. Iseler in an article entitled "Efficient Frequency Tripling of CO.sub.2 -Laser Radiation in Tandem CdGeAs.sub.2 Crystals" in Optics Letters, Volume 4, page 55 (Feb., 1979) describe frequency tripling of CO.sub.2 -laser radiation using CdGeAs.sub.2 crystals to produce second-harmonic generation in one crystal followed by sum mixing of the fundamental and second harmonic in a second crystal.
However, in the use and application of such technologies for the detection of gases in the atmosphere wherein a number of different gases or mixtures may be present, the need still remains for a system capable of more flexibility. For example, the CO.sub.2 laser systems in use emit radiation at 80 different wavelengths in the 10 micron region. While frequency tripling techniques will provide 80 wave-lengths in the 3 micron region, a region that is more usable in detection systems due to the lower attentuation, the number of wavelengths available is still too small to provide a useful spectral match with some gases. One needs a spectral pair for each gas to be detected. Furthermore it is often necessary to have more than one pair since some pairs may not be useful if other gases are also present. This is because some of these gases may intefere by having differential absorption coefficients at a particular pair of wavelengths that are sufficiently large so that it is impossible to distinguish these gases from each other. Thus it is desirable to have a large number of wavelengths available. For example, detection of methane by itself presents no problem. However, the difficulties can arise if one is attempting to measure methane in the presence of a number of other gases, as might be true when searching for a natural gas leak with various hydrocarbons emanating from the same source. Another example is petroleum exploration in which various other gases, both hydrocarbons and air pollutants, are potential interferants. It would, therefore, be highly desirable to have a system having available many more wavelengths from which to select wavelength pairs for analysis of a multitude of gases present either singly or in combination.